1. Field of the Invention
The invention relates to the technical field of decoding and, more particularly, to a decoding system for eight-to-fourteen modulation or eight-to-sixteen modulation (EFM/ESM).
2. Description of Related Art
In an inter-symbol interference (ISI) environment, there is an effect of read data due to previous and subsequent data bits, a partial response maximum likelihood (PRML) technique is typically used to eliminate the ISI effect occurring in received data. Typically, the Viterbi decoding is used to decode a signal with the ISI.
The Viterbi decoding is based on conditional probability to check possible paths to accordingly select the best one. FIG. 1 is a typical PRabba channel model. FIG. 2 shows a schematic diagram of Trellis corresponding to the typical PRabba channel model of FIG. 1 for a=1 and b=2.
Typically, an eight-to-fourteen modulation/eight-to-sixteen modulation (EFM/ESM) is used for DVD data and CD data. Thus, the run length of a datastream excludes one and two. The Trellis diagram of FIG. 2 can be simplified into a Trellis diagram of FIG. 3 when an EFM/ESM datastream is decoded. As shown in FIG. 3, the run length of a datastream excludes one and two, and dotted lines indicate a4 as −1, solid lines indicate a4 as +1. FIG. 4 is a schematic diagram of a Viterbi decoding performed on an input sequence {−1, −1, −1, −1, +1, +1, +1, −1, −1, −1, −1, +1, +1, +1, +1, +1} with the simplified Trellis of FIG. 3 and to thus generate an output sequence {4, 0, −4, −6, −4, 0, 4, 4, 0, −4, −6, −6, −4, 0, 4, 6, 6, 6}. The decoding starts with node (−1 −1 −1) and computes branch metric of rows 0 and 1 in accordance with a received data. Node (−1 −1 −1) has two branches connected to nodes (−1 −1 −1) and (−1 −1 +1) respectively. For computing branch metric from node (−1 −1 −1) to node (−1 −1 −1), an output data (4) is brought into path value equations, i.e., s=4 is substituted into equations 9+3s and 4+2s to thus obtain values 21 and 12 respectively. The values 21 and 12 are denoted on the upper of nodes (−1 −1 −1) and (−1 −1 +1) of row 1. For computing branch metric from row 1 to row 2, s=0 is substituted into equations 9+3s, 4+2s and 0 to thus obtain values 9, 4 and 0 respectively. Next, values 9 and 21 are added, values 4 and 21 are added, and values 0 and 12 are added respectively, thereby obtaining accumulated branch metrics 30, 25 and 12. The values 30, 25 and 12 are denoted on the upper of nodes (−1 −1 −1), (−1 −1 +1) and (−1 +1 +1) respectively, and similar operation is repeated. However, the read speed of DVD and CD drives is increased quickly, and the prior art increases a clock of a decoding chip to overcome the increase of read speed, but it will also increase the design difficulty of the decoding chip. In addition, due to the increased operating frequency, the decoding chip needs to consume more power and has the problem of heat dissipation.
Therefore, it is desirable to provide an improved decoding system to mitigate and/or obviate the aforementioned problems.